fix return type of writing into monotonic heap
fix return type of writing into monotonic heap

akuhlens/schml,Gradual-Typing/Grift,Gradual-Typing/Grift,akuhlens/schml,akuhlens/schml,akuhlens/schml,akuhlens/schml,akuhlens/schml,Gradual-Typing/Grift,Gradual-Typing/Grift